STRIPED BASS REBUILDING: THE REMAINING UNCERTAINTIES

Last Sunday, I released what some might consider an uncharacteristicallyoptimistic essay that predicted that there will be a striped bass rebuildingplan in place for most—and, in many states, all—of the 2023 fishing season, andthat such rebuilding plan will require fishing mortality reductions that, whilesubstantial, will be small enough to make rebuilding by the 2029 deadline avery realistic goal.

However, I also noted that the data discussed at last week’s Atlantic Striped Bass Technical Committee meeting was preliminary, that there would be more analysis done, and that the precise magnitude of the needed fishing mortality cuts had yet to be determined, although such reduction might well be somewhere around 25%.

Those comments led to some questions by readers, including some readers who are very familiar with the fishery management process.  Thus, I thought that today’s edition of One Angler’s Voyage ought to describe the remaining uncertainties in a bit more detail.

Uncertainty in the striped bass stock assessment update flows from two primary sources.  One is a retrospective pattern in the estimates of fishing mortality and spawning stock biomass, which only becomes apparent when additional years of data are added to the original estimates.  The other is the selectivity of the gear used in both the fishery-dependent and the fishery-independent surveys used to gauge striped bass abundance, and how such selectivity impacts estimates of spawning stock biomass.

I’ll address the retrospective pattern first.

Typically, in just about every stock assessment, the data relating to the assessment’s terminal year are the most uncertain.  As time goes on, and a stock assessment update provides additional years of data, the estimates of fishing mortality and spawning stock biomass for what had been the terminal year often changes.  If such change is consistent in direction and approximate magnitude across many years’ data, a “retrospective pattern” emerges.

Such pattern can, in some situations, be significant.  Fortunately for striped bass, the retrospective pattern produced by the current assessment model is relatively minor. 

The 2018 benchmark stock assessment notes that

“Very little retrospective trend (+/-2%) was evident in the more recent estimates of fully-recruited total [fishing mortality], female [spawning stock biomass], and age 8+ abundance [in the structured catch-at-age model]…Approximately 5 years of additional data are needed before the percent-difference from 2017 estimates increases to +/1 10 to 15%...The retrospective pattern suggests that fishing mortality is likely slightly over-estimated and could decrease with the addition of future years of data.”

Four years have passed since that was written, and the preliminary stock assessment update data does, in fact, show a small retrospective pattern in the data from 2017 and immediately prior years.  However, such pattern does not indicate that fishing mortality in 2017 had been slightly overestimated.  Instead, it indicates that the conclusions in the benchmark assessment were slightly optimistic; fishing mortality was actually somewhat higher, and female spawning stock biomass somewhat lower, than indicated in the 2018 benchmark.

While the differences between the benchmark stock assessment and the pending update don't appear to be large, the Technical Committee still needs to decide whether the retrospective pattern is significant enough to consider when recommending rebuilding measures to the Management Board.  

Retrospective patterns have long appeared in striped bass stock assessments, but have generally not been given much, if any, weight when management measures were crafted.  Given the relatively small pattern that is emerging in the assessment update data, that trend will probably continue, although given the overfished state of the stock, it is possible, if not likely, that the Technical Committee will want to adjust for the retrospective pattern in order to provide for a more precautionary management approach.

Selectivity presents a somewhat more complex issue.

In preparing the benchmark stock assessment, biologists used data from 13 different surveys to create indices of relative striped bass abundance, as well as the relative abundance of the various year classes composing the bass population.  12 of the 13 surveys were fishery-independent surveys conducted by various state natural resource agencies.  Data developed by the Marine Recreational Information Program, sampling of commercial landings, information developed by various tagging programs, and other data is also taken into consideration.

One of the big questions that the scientists have to consider is whether the various surveys and other sources of data sample a relatively equal proportion of fish from each year class, or whether some cohorts are over- or under-represented.  It’s not an easy question to answer, as different fisheries often target particular cohorts, and such cohorts don’t always utilize the entire range of the striped bass population.

Thus, immature striped bass dominate both the population and the fisheries in the Chesapeake Bay, while adults play a far larger role in coastal regions, particularly in the northeast.  

Historically, the selectivity curves used in striped bass assessments include a “domed” selectivity in the Chesapeake Bay, which shows a sharp peak indicating very high sampling of the younger year classes that dominate both the Bay's striped bass population and its striped bass fisheries, then tapers off sharply as older fish enter the coastal migratory population, and a flat-topped curve on the coast that ramps up sharply as maturing bass enter the migratory population, then flattens out and remains generally constant once such fish grow into the 28-inch minimum size that has long been in place for most states' commercial and recreational fisheries.

The problem with the latter curve is that Amendment VI to Amendment 6 to the Atlantic Striped Bass Interstate Fishery Management Plan upset historical norms by eliminating the minimum size in recreational fisheries, and imposing a 28- to 35-inch slot limit on most of the coast, although a couple of states were permitted a slot limit with a top end that was an inch or three higher.  Such measure was put in place beginning in 2020, and may well have altered the selectivity curve for the coast.

While the measure would not have impacted the fishery-independent surveys, it would have substantially altered the data developed by the Marine Recreational Information Program, since fish over 35 inches could no longer be retained by anglers along most of the coast, while bass over 38 inches could only be kept during the few weeks that Maryland’s so called “trophy season” was open in the Chesapeake Bay.

So while data from the Chesapeake Bay still seems to fit the domed selectivity curve used in the past, an argument can be made that, for the years 2020-2021, the flat-topped selectivity curve used for the coast should be replaced with a domed curve that reflects the lower proportion of large striped bass appearing in MRIP data.

The use of a domed selectivity curve implies that there are more large striped bass in the population than indicated by the existing data.  That implication forces the Technical Committee to use their professional judgment to make a decision:  Are fewer large striped bass showing up in the data because the selectivity has changed, or are fewer striped bass showing up in the data because they are also absent from the population as a whole?

Arguments can be made for both propositions.  There is no question that the selectivity of the recreational fishery-dependent survey has changed, at least with respect to the coastal fishery.  The question that the Technical Committee will probably have to decide is whether the decline of older, larger fish reflected in the MRIP data is also reflected in the fishery-independent surveys.  If such surveys don’t show a similar decline, then a new selectivity curve for 2020-2021 is probably justified; on the other hand, if the fishery-independent surveys also show a similar decline in large bass, such fish probably also make up a small part of the overall population.

And, as is often the case with fisheries issues, the right answer is probably somewhere in between.  

The Maryland juvenile abundance index, which has historically been the best single gauge of striped bass abundance, indicates that, over the past 25 years—few, if any, bass will live longer than that given the current level of fishing mortality—notably large year classes were produced in 2001, 2003, 2011, and 2015, with smaller, but still well above average, year classes produced in 1999 and 2005.  Many of the years in between saw recruitment that was significantly below average.  Given that the largest of the big year classes, 2001, is now two decades old, and that most of the other “large” year classes were about half of its size, the number of large bass in the population must be relatively low.  At the same time, whatever big bass remain were probably underrepresented in the 2020-2021 recreational data.

So the Technical Committee will have to resolvethe selectivity issue.  If it decides that because of domed selectivity, the number of large bass present in 2020-2021 was significantly under-sampled, then it follows that the size of the fishing mortality reduction needed to rebuild the spawning stock by 2029 will be less than it would be if the Committee decides that selectivity in those years is a relatively minor issue.

Right now—and as new information develops, the situation could change—the manner in which the Technical Committee resolves the retrospective pattern and selectivity issues will result in a rebuilding plan that calls for a fishing mortality reduction somewhere between 9 and 27% (there is another scenario that would eliminate the need for any rebuilding plan at all, and allow for a 17% increase in landings, but that one seems to be an outlier that will not be seriously considered).

While there is already a certain amount of grumbling that the Technical Committee will open the door to Management Board efforts to adopt half-measures that won’t rebuild the stock, I don’t believe that will happen.

The Technical Committee is composed of capable and experienced fishery scientists, who have always demonstrated both competence and integrity, along with a desire to provide the best scientific advice possible.  I have confidence that their final recommendation will, in their professional judgment, reflect the current state of the striped bass stock and provide good advice on how to move forward.

Whether the Management Board will take their advice is another, very different question.  Still, given how most Management Board members have responded to both the state of the striped bass and the concerns of stakeholders throughout the Amendment 7 process, I hope and believe that they will take the correct actions, too.

Time will tell.